Dermoid Sinus in Ridgebacks: When the Ridge Carries a Risk

Both trace back to one mutation: a duplication of about 133 kilobases on canine chromosome 18 (CFA18) covering the fibroblast growth factor genes FGF3, FGF4 and FGF19 plus ORAOV1. In 2007 Salmon Hillbertz and colleagues showed that this duplication causes the ridge in Rhodesian and Thai Ridgebacks and interpreted it as a predisposition to dermoid sinus as well; the breakpoint is identical in both breeds, so the mutation shares a common origin. FGF signalling steers early development, and the leading hypothesis is that altered gene dosage during skin development explains both the reversed hair direction and the incomplete closure.

Embryologically, the neural tube – the future brain and spinal cord – forms when the ectoderm folds in and then detaches from the surface ectoderm that becomes skin. If that separation is incomplete, a tract lined with stratified squamous epithelium remains, which may contain hair follicles, sebaceous and sweat glands. In Ridgebacks the typical sites are the dorsal midline of the neck, the front of the chest region and the sacrococcygeal area – in front of or behind the ridge, rarely within it.

Classification is by depth, going back to Mann and Stratton (1966): type I reaches the supraspinous ligament, type II stops short but is tethered to it by a fibrous strand, type III ends superficially with no connection, and type IV reaches the dura mater, leaving an open path into the vertebral canal. Type V (a cyst with no skin opening) and type VI were added later; type VI is used for rare, inconsistently defined forms. One dog can have several sinuses of different types.

Pathologically the tract fills with keratin, sebum and hair, leading to inflammation, abscesses and draining tracts. Type IV sinuses risk myelitis, meningitis or meningomyelitis, with fever, pain, hyperaesthesia, ataxia and paresis up to paralysis. Diagnosis rests on palpation, supplemented where needed by contrast radiography, CT or MRI, and is confirmed by histology. Treatment is complete surgical excision of the entire tract including the fibrous strand – with a laminectomy when the dura is involved.

An important caveat: a large analysis of German breeding records (Distl 2022) found a heritability of 0.78 but no simple inheritance pattern, and considered it unlikely that the ridge locus alone determines whether dermoid sinus appears. A genetic study the same year concluded that the ridge duplication is not identical with the hypothesised dermoid sinus locus (Zhang & Distl 2022). Ridgebacks do carry a raised risk, but which genes cause it is still unresolved.

By a breeder who treats dermoid sinus as a real risk: every puppy has been palpated several times along the whole back midline, the parents' ridge genetics are known, and the mating was planned to keep the risk low. A breeder who says it never happens in their lines should be able to back that up with figures.

Palpation is hands-on work: the skin is lifted along the midline from the back of the skull to the base of the tail and checked between thumb and forefinger. A dermoid sinus feels like a thin cord pulling downward. Many vets rarely see the condition, so it is worth asking for this check specifically.

Because a trait the breed standard requires is genetically linked to a defect. The ridge itself causes the dog no pain, and the great majority of Ridgebacks never develop a dermoid sinus. The question is how dogs are bred: selecting only for the ridge increases the share of dogs with two copies (R/R) – and a higher risk has been reported for them.

In the 2007 study, about 87% of the Ridgebacks with dermoid sinus were homozygous (13 of 15), compared with roughly a third of healthy ridged dogs. These are small numbers. Later work (Distl 2022; Zhang & Distl 2022) suggests that the ridge gene alone does not explain dermoid sinus. Not pairing two homozygous dogs is therefore mainly a precaution.

Frequency has come down. Earlier Swedish estimates put affected puppies at 8 to 10%; in Germany the figure was 2.5% of 12,700 puppies between 2001 and 2019 and falling, and Swedish breeders reported around 2% for 2015 to 2019.

With respect to dermoid sinus they are the safest group, yet the standard sorts them out. FCI standard no. 146 lists dogs without a ridge as a disqualifying fault, so they cannot be shown, and many clubs exclude them from breeding. Ridgeless puppies do not carry the duplication; dermoid sinus occurs in them only in rare individual cases.

Healthy ridgeless puppies were historically culled in some places – this is documented in the scientific literature. Today club rules such as the code of ethics of a UK Ridgeback association say these puppies should be rehomed as pets, with a contract to spay or neuter and not to show or breed them. In Swedish litters from 1981 to 2002, ridgeless puppies were estimated at 5.6%.

This is the heart of the dilemma: every ridgeless puppy removed from breeding takes with it a gene variant that could lower the risk in the next generation. Researchers have therefore suggested pairing homozygous dogs only with heterozygous ones, or deliberately bringing ridgeless dogs into breeding. Which path prevails is up to the breed clubs.

No – there is no explicit ban on the ridge or on either Ridgeback breed, and German law does not name specific breeds. Section 11b of the German Animal Welfare Act prohibits breeding dogs where the offspring can be expected to suffer inherited pain, suffering or harm; whether that applies is assessed individually, based on the animal's traits. Owning an affected dog is not prohibited.

To interpret the rule, German authorities use the expert opinion on Section 11b (the ban on extreme breeding), usually dated 1999. It is an interpretation aid, not binding law – but it names dermoid sinus in the Rhodesian Ridgeback explicitly and recommends a breeding ban for dogs with dermoid cysts, not for the ridge as such.

In Switzerland, the ordinance on animal welfare in breeding lists dermoid cysts in the Rhodesian Ridgeback among the traits that may cause a medium or severe burden; breeding from such a dog there requires a prior burden assessment by a qualified person.